1. Field of the Invention
The invention relates to scanning apparatus which may be used in a real-time imaging system and, in particular, in a real-time passive millimetre wave imaging system. The scanning apparatus may also be used in other radiometry systems.
2. Discussion of Prior Art
British Patent No. 700868 (February 1952-December 1953) describes a twistreflector which relates to a similar field as the present invention.
Millimetre wave imaging is potentially useful as an all-weather surveillance and guidance aid but any practically useful system must be capable of imaging in real-time. This is not possible using existing systems. In a millimetre wave imager, radiation from the scene to be scanned is collected by means of a concave mirror or a lens and is focused onto an array of millimetre wave receivers. At present, large two-dimensional arrays of receivers which cover the whole of a required image are not available. Instead, a far smaller number of receivers is scanned across the image in order to build up the complete picture. A similar technique is used in some infrared imagers (for example EP 0226273).
Current millimetre wave imaging systems use mechanical scanning of one or several channels to synthesise an image. Ultimately, electronic scanning and staring array techniques could be developed to implement real-time millimetre wave imaging, although there are several problems associated with such a solution. Firstly, as the wavelength is necessarily long, in order to image under adverse weather conditions the system aperture must be large to gain adequate resolution. In some millimetre wave imaging systems the input aperture may be of the order of 1 m in diameter. Secondly, the cost per channel is high so that any electronically scanned or staring array technique is expensive. Furthermore, in the case of millimetre wave staring arrays there are fundamental problems analogous to the cold shielding problems encountered in infrared systems.
Another requirement of a practical millimetre wave imaging system is that it must be able to operate at TV-compatible rates (i.e. 50 Hz for the UK, 60 Hz for the USA). In the infrared, scanning systems are often plane mirrors flapping about an axis contained within their surface. This is not a practical option in the millimetre waveband as large aperture mirrors would be required to flap back and forth at TV-compatible rates, requiring a large change in inertia at the end of each scan.
In infrared imaging systems, where input apertures are typically only 10 mm in diameter, rotary systems have been used (EP 0226273). Furthermore, in the infrared, it is usual to employ afocal telescopes to match the field of view in the scene to that of the rotating polygon. This is impractical in high resolution millimetre wave imaging where the input apertures have considerably greater diameters and afocal telescopes would need to be excessively large.
Any scanning mechanism used in a millimetre wave imaging system must therefore be situated in either the object or the image plane. Furthermore, any scanning mechanism situated in the image plane must have good off-axis performance. This is difficult to achieve using existing technology.
Another known scanning method used in infrared imagers is a system of two discs rotating about axes which are slightly inclined to the normals to their faces. Radiation incident on the first disc is reflected at oblique incidence from the first rotating disc and passes to the second disc to experience a second reflection. By varying the orientation and relative speed of rotation of the discs, varying scan patterns can be achieved. Such a two-axis rotating disc system would not be ideal for use in millimetre wave imaging, however, as the system would be inconveniently large.
It is an object of the present invention to provide a compact object space scanning apparatus which may be used, in particular, to implement real-time millimetre wave imaging, or in radar systems. It is also an object of the invention to provide a scanning apparatus which has limited power requirements and minimum inertia and gives good off axis performance.
According to the present invention, apparatus for scanning radiation from a scene and for generating output radiation for input to a receiver system comprises;
a first rotatable reflective plate, for receiving and reflecting radiation, having an axis of rotation passing substantially through the centre of the plate, wherein the axis of rotation is inclined at a non-zero angle .theta..sub.a to the normal to the reflective plate, PA1 rotary means for rotating the reflective plate, PA1 secondary reflection means for receiving and reflecting radiation and PA1 static reflection means for receiving radiation reflected from the first rotatable reflective plate and reflecting radiation towards the secondary reflection means, PA1 characterised in that the secondary reflection means is a second rotatable reflective plate having a common axis of rotation with the first rotatable reflective plate, wherein the common axis of rotation is inclined at a non-zero angle .theta..sub.b to the normal to the second reflective plate. PA1 wherein the polarisation axes of the polarisers forming each roof reflector are oriented to transmit radiation having substantially the same direction of polarisation wherein said direction of polarisation is substantially parallel or substantially perpendicular to the apexes. PA1 such that the radiation having a particular direction of polarisation may be output through the polarising roof reflector. PA1 whereby the one or more birefringent surfaces introduce a substantially 90.degree. phase shift in the state of polarisation, P.sub.s, each time radiation passes through the one or more birefringent surfaces, such that radiation having a particular direction of polarisation may be output through the one or more polarising roof reflectors. PA1 a second Faraday rotator for rotating the direction of polarisation of radiation output from the polarising roof reflector through substantially 45.degree. PA1 and may also comprise a second polariser, wherein the second polariser has an axis of polarisation inclined at substantially 45.degree. to the apex of the polarising roof reflector. PA1 a first polarising surface having a polarisation axis, for selectively transmitting and selectively reflecting radiation having a particular direction of polarisation, PA1 a second surface for rotating the direction of polarisation of radiation through substantially 45.degree. and PA1 a third polarising surface for selectively reflecting and selectively transmitting radiation, PA1 wherein the third polarisation axis makes an angle of substantially 45.degree. with the first polarisation axis.
The scanning apparatus provides the advantage of compactness. It has minimum inertia and minimum power requirements. The apparatus may be situated at the entrance pupil of an imaging camera or receiver and provides good off-axis performance.